1. Field of the Invention
The present invention relates to a phase shifting device for manipulating vector-displayed waveforms in a phase advancing or retarding direction, i.e., a direction of rotation on a vectorscope, and more particularly to a phase shifting device for a vectorscope which is used to manipulate an entire vector-displayed waveform such that a particular desired portion of the vector-displayed waveform is located at a predetermined target phase (or angular) position on vector display coordinates or on a vector display graticule.
2. Description of the Prior Art
Conventional vectorscopes are provided with a phase shifter which shifts in phase a subcarrier regenerated from a color video signal in order to rotate an entire vector-displayed waveform on vector display coordinates. It should be noted that in the vector display coordinate, the angle and magnitude of each vector in the vector display coordinates represent hue and saturation, respectively. The phase shifter is responsive to a manual operation of a phase shift knob on a control panel to successively shift in phase the regenerated subcarrier by any desired amount in open loop control such that the vector-displayed waveform is rotated from the currently displayed phase position.
Japanese Patent Appln. Public-Disclosure No. 5-122734 (1993) discloses a vectorscope automatic burst positioning circuit which shifts in phase, in response to operation of an auto burst position button, a vector-displayed waveform from a currently displayed phase position to a target phase position substantially successively by an unspecified amount corresponding to the difference between the currently displayed and target phase positions. For this purpose, the automatic burst positioning circuit generates in closed loop control a phase shift control signal which is inputted to a phase shifter for phase-shifting of a regenerated subcarrier. More specifically, predetermined conditions are defined for representing that a particular portion of a vector-displayed waveform, i.e., a burst vector has reached a target or reference phase position (at 180.degree. position in the NTSC standard), and the phase shift control signal is incremented until the conditions are satisfied. The predetermined conditions may be defined with respect to the values of R-Y and B-Y signals which are chrominance components sampled during a burst interval.
The phase shifting methods employed in the conventional vectorscopes give rise to problems when an operator attempts to rapidly and accurately rotate a vector-displayed waveform in order to move any predetermined portion of the vector-displayed waveform to any target phase position. Specifically, the above-mentioned first method using the phase shift knob requires a certain amount of time since phase shifting must be performed by progressively rotating a displayed waveform. The method also requires the operator to visually confirm whether a predetermined waveform portion, which is a reference for phase shifting, accurately matches with a predetermined target phase position. If the matching is not accurate, fine adjustments must be carried out. Thus, the method is problematic in that a rapid and accurate phase shift operation cannot be performed easily.
The second phase shifting method using the automatic burst positioning circuit suffers from restraints due to the circuit configuration. Specifically, a waveform portion to be selected as a phase shifting reference and a target phase position cannot be arbitrarily selected or changed, and a particular portion of a vector-displayed waveform, i.e., a burst vector portion is only regarded as a phase shift reference, and a target phase position is fixed to the standard phase position of the burst vector. The positioning circuit is provided with a sample and hold circuit for sampling and holding R-Y and B-Y values during a burst interval and a zero adjust circuit for determining conditions which are satisfied by the R-Y and B-Y values when a burst vector signal is shifted to the standard burst phase position. Further, the closed-loop operation of the auto burst positioning circuit, although achieving accurate phase shifting, is very slow because determination is made as to whether a burst signal vector has reached the standard phase position each time a phase shift is incremented (or decremented).